1. Field of the Invention
The present invention relates to image processing apparatuses and methods, and to storage media used therewith.
2. Description of the Related Art
Recently, image forming apparatuses, such as color printers and color copying machines, can form high quality images because the apparatuses have improved in performance. Due to the improvement, the apparatuses are becoming able to form images similar to those of bank notes, securities, etc. Accordingly, it is likely that problems such as forgery of bank notes and securities, and piracy of copyright, will increase in future.
As one of prevention, an addition method is known in which, for example, a dot pattern indicating the serial number of an image processing apparatus is added to a color image printed by the image processing apparatus so that it is imperceptible to human eyes.
Normally, the dot pattern has a predetermined size, and a plurality of dots are arranged in the predetermined size. Additional information can be represented by the arrangement of the dots. The dot pattern is repeatedly printed on the entire area of the image. In order that the dot pattern may be imperceptible to human eyes, when the dot pattern is added to a color image composed of yellow, magenta, cyan, and black planes, it is normally added only to the yellow plane.
By performing the above-described addition of the dot pattern, when an image that is not allowed to be formed or an image that is not allowed to be reproduced appears, the added dot pattern is extracted and additional information (a serial number) represented by the pattern can be decoded, whereby the apparatus used to form the image can be determined.
An example of a conventional 1-dot pattern is shown in FIG. 1. In FIG. 1, each square indicates a pixel formed by an image forming apparatus. Each black square is a pixel which bears the color. Each white square is a blank pixel. Other pixels (not shown) are used to display information based on image data.
One image forming unit or one row of image forming units such as lasers and light-emitting devices are used in a conventional image forming apparatus. However, it is likely that at least two image forming units or at least two rows of image forming units will be used in order to accelerate the image formation. In this construction, in order to add conventional dot patterns, a specific pattern adding unit must be provided for each image forming unit. This inevitably increases the production cost.
The conventional dot pattern causes the following problems. First, if the dot pattern is formed in yellow, it can be easily seen, particularly when it is highlighted. This causes deterioration in image quality. Second, a dot pattern which is formed of dots generated by dithering or the like cannot be extracted since it is superficially covered with the generated dots. Third, due to precision of component units for synchronizing the start positions of lines constituting an image, the positioning precision of each laser, etc., each line may be shifted in position in the a main-scanning direction. This positional shift greatly affects a conventional dot pattern, so that the dot pattern cannot be decoded.